Hi Garbarrage you must be getting sensitive a feeling your ride more. That's good.
The overall fin size will create a little more drag in paddling weigh this up against the bite of the fins in the steep part of the face , downsizing carefully can get you a finer tuning and more short arc turn ability , but at increased risk of sideslipping the tail but if you are riding a short board this is what the pros and good riders factor in for their big turns.
Longboards a different and more difficult thing to control. Remind me of what you are riding? A way to get the little extra in the take off speed is to change you timing with the wave and be in a slightly more critical spot, it requires a shorter faster burst of paddling speed, but gets you through that prolonged paddle where you run out of puff just before that final stroke.
On either board type if in the short critical take off you can use what I call the reverse cork technique where just before you paddle you push your board backward and as deep into the approaching face as possible, then let the buoyancy rebound give you a booster to that first paddling power.
You can also add a little finesse past that to the take off with a strong push down on the board with the pop up push up
driving the nose down the face to pick up gravity speed, explaining why short boards for steep waves have that increased nose rocker , just a slightly more difficult judgement to make on a long board.
Some great successes will be had with these things and your friends will also enjoy your spectacular and epic FAILS as you go for it.
Finally a simple rule of take offs: IF you are going to go for it COMMIT TO IT
if you are not sure let it go someone else should have it.
These guys are committed or ought to be check out this little videohttp://www.youtube.com/watch?v=6O9qAyjGDLs
BTW Uncle Jaffa's advice of any sort doe not apply under these circumstances, I've never gone that big
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun!